A Closer Look: The Electron Structure of Water and its “Shape”

Our best understanding of electrons: Quantum mechanics

In order to understand
what it means for a particle to have a shape and how this affects its
macroscopic properties, we must first refine our
idea of what an electron is.

When subatomic particles, including the
electron, were discovered, they were assumed to be tiny spheres — in
fact, they were assumed to be so tiny that they didn’t have a diameter
at all. In the early 20th century, however, more refinements were made
in the theory of the behavior
of very small things, and it was discovered that the world at the microscopic
level is not as clearly defined as it seems to be at the macroscopic
level.

Rather than calculating "paths" of electrons within
atoms or molecules, we calculate the probability of finding the electrons
within
various parts of the space around the nuclei. Scientists describe
the area around a nucleus where they expect to find electrons a “probability
cloud.”

It turns out that when two atoms share or exchange electrons,
the shape of the “cloud” where you would expect to
find an electron changes shape. Although a single hydrogen atom
has an
electron
cloud which looks
like a sphere, [AP, Mark says “SHOW THIS] when it shares
its electron with an oxygen atom, the overall shape of the cloud
changes,
forming a “V.”

How are these “electron clouds” related
to the forces between particles?

Recall what was said in this
session’s video about the electrostatic
force: positive charges and negative charges attract one another.
The electron cloud in a molecule or atom is negatively charged. The nucleus
of an atom
or the nuclei of a molecule are positively charged because
they contain positively charged protons. The electron cloud stays fairly
close
to the
nucleus because there is an attractive force between them.
However, when there are many atoms or molecules of the same substance,
there is
also
an attractive force between the electron cloud of one particle
and the nucleus (or nuclei) of its neighboring particles. This electrostatic
attractive
force is the “force between particles” referred
to throughout this series.

It is the shape of the particle,
i.e., the shape of its electron
cloud, which determines the strength of the force between
particles. To clarify
further, let’s look at two examples of the shape of
molecules, water and carbon dioxide.

Water

The oxygen atom at the “corner” of the water molecule
(H20) has a particularly strong pull on the electrons of the hydrogen
atoms. As a result, the probability of finding the electron closer
to the hydrogen
atom is reduced. Because of this, the two “prongs” of
the water molecule are slightly more positively charged
and the “corner” is
more negatively charged. We call this kind of molecule
a “polar” molecule
because the electrons are shared unevenly, resulting
in an uneven shape:

The
uneven charge distribution, i.e., the shape of the electron cloud, also
results in a strong force between
water molecules,
which explains
why water is a liquid at room temperature and has a
relatively high boiling point.

Carbon Dioxide

The shape of the carbon dioxide molecule, C02,
is linear. The carbon atom at the center does not have a
particularly
strong
pull on
the electrons from the two oxygen atoms on either side.
In this case,
there is not
a
negatively charged side and a positively charged side,
as there is in a water molecule. As a result, the forces
between
the
molecules of carbon
dioxide are weaker than those between water molecules.
Thus, when particles of carbon dioxide, at room temperature,
are
moving about
and colliding,
they cannot “hold on” to each other, explaining
why carbon dioxide is a gas at room temperature.